Probe cards or testing devices are crucial for efficient manufacture of electronic circuits. These devices enable one to test and isolate defective circuits during production. Probe cards are frequently employed for functional testing of integrated circuits on wafers before cutting and mounting them inside an IC chip package. The arbitrary positions and enormous number of contact pads in such circuits, especially in the very large scale integration domain, impose stringent requirements on probe cards.
In particular, the probe card's contacting elements, probe needles or probes are affected by these conditions. The probe is repetitively driven against the pads on wafers under test or C4 solder bumps of electrical circuits. The distance by which the probes are moved towards the pads is commonly called overdrive. When driven against the pad the probe undergoes a deflection and its tip portion executes a lateral movement. The lateral scrubbing of the tip helps to remove an insulating oxide layer formed on the surface of the pad. This ensures proper electrical contact between the probe and the pad. Otherwise, the contact resistance between the probe and the pad would prevent the passage of electrical signals necessary for testing.
Furthermore, the repetitive nature of the testing process, geometrical unevenness of wavers, abrasion and fatigue of probe tips all affect the long-term probe performance. In particular, these factors affect the horizontal alignment or planarity of the probe tips and prevents the establishment of proper electrical contact between the probes and the corresponding pads. Large variance in planarity can not be overcome by increasing the overdrive since this would damage or even destroy the probes. Thus, planarity should be preserved at all times.
It has been recognized that one of the most effective ways to preserve planarity and ensure long life of probes is to ensure that the contact force F between the probes and the pads should be equalized. In other words, all probes should preferably experience the same contact force F with the pads.
Attempts have been made at solving these problems by suitable probe mounting and design. In U.S. Pat. No. 5,334,931 Clarke et al. present a probe formed from a molded plastic and equipped with a conductive contact tip. The body of the probe is cantilevered and designed such that the contact tip scrubs the surface of the pad of a device under test when overdrive is applied. Although this construction makes replacement of the conductive contact tip simple, the mounting arrangement is complicated, and planarity can not be ensured after many testing cycles. Moreover, this design does not ensure that the contact force F between the probes and the pads is equalized.
In U.S. Pat. No. 5,280,236 Takahashi et al. propose a probe made of a cobalt-based alloy containing at least 10 wt. % of chromium. The probe has a solder-enhanced metallic layer on its other end. These provisions ensure good scrubbing action due to the metallic coating on the tip and probe longevity due to the elasticity of the probe itself. Nonetheless, the application of repetitive stress disturbs probe planarity and deforms the probes.
Finally, in U.S. Pat. No. 4,980,638 Dermon et al. present a probe of controlled shape and dimensions. The shaft portion of the probes is tapered for easy replacement of probes. The probes are etched out of a sheet. There are no provisions to ensure longevity of the probes or sufficient planarity.
Subramanian describes a high density probe card for testing electrical circuits in U.S. Pat. No. 5,382,898. Although the structure of this device is well-adapted to high frequency ranges and high pad densities it does not address the issue of equalizing the contact force.
Ueno et al. in U.S. Pat. No. 5,491,427 teaches how to prolong the life of a probe by adjusting the alignment of the probe electrodes and the device to be probed. In particular, these inventors focus on various geometrical arrangements of the probes. The preferable arrangement of probes, according to their teaching, is in columns. In addition, Ueno et al. accept a certain fail ratio of probes by providing a novel grid pattern of electrical interconnections which allow one to continue probing while avoiding the failed probes. While this approach is advantageous from many points of view, it is not directed at the root problem of contact force equalization. Instead, defective and failed probes are shorted out of the electrical circuitry instead, thus treating the symptoms rather than the underlying problem.
All of the above-mentioned prior art probes suffer from long-term stress fatigue and lack of sufficient equalization of the contact force between the probes and pads of the circuit under test. Consequently, planarity is lost and, in more severe cases, probes break under stress concentrating at their attachment or support points.